Until quite recently, it has not been advisable to use glass fibers for long term (five or more years) reinforcement of hydrous calcium silicate crystals, cement, concrete, mortar or other cementitious materials or matrices that have a high alkali content. The harsh alkali environment would degrade the types of glass fibers, E glass for example, commonly used to reinforce non-alkali materials such as plastics.
E glass fibers are not generally recommended for the long term reinforcement of Portland cement or other cementitous products. The alkali content of the cementitious matrix attacks the E glass fiber surface and substantially weakens the fibers. This alkali attack and subsequent fiber strength loss generally so weakens the fiber that long term reinforcement of the matrix by the E glass fibers is neither predictable nor dependable.
To remedy this situation, the prior art has tried a number of potential solutions. One is to coat the fibers with some material that is alkali resistant. Epoxy resin coated fibers, for example, generally will withstand alkali attack. Another potential solution is to use a high alumina cement which has less alkali content. Still another solution is to formulate a glass composition which in fiber form will be resistant to alkali attack.